Chemokines are a large and growing family of 6–14 kD (non-glycosylated) proteins that mediate a wide range of biological functions (Taub, D. D. and Openheim, J. J., Ther. Immunol., 1:229–246 (1994)). The chemokines can be divided into families based on the position of four cysteine residues that form two disulfide bonds (Kelner, G. S., et al., Science, 266:12395–1399 (1994); Bazan, J. F., et al., Nature, 385:640–644 (1997); Pin, Y., et al., Nature, 385:611–617 (1997)). Chemokine receptors can also be divided into families based on the type of chemokine they bind, although, no clear structural differences have been identified that distinguish the receptor sub-families (Mackay, C. R., J. Exp. Med., 184:799–802 (1996)). In addition, there are a number of so-called “orphan” chemokine receptors (e.g., Bonzo) which share sequence homology with well-characterized chemokine receptors, but for which the biological functions and specific receptor agonists remain unknown.
Chemokines play a vital role in leukocyte adhesion and extravasation. For example, in various in vitro assays, chemokines can induce the chemotaxis or transendothelial migration of leukocytes (Taub, D. D. and Oppenheim, J. J., Ther. Immunol., 1:229–246 (1994)), while in vivo injection (Taub, D. D., et al., J. Clin. Invest., 97:1931–1941 (1996)) or over-expression of chemokines (Fuentes, M. E., et al., J. Immunol., 155:5769–5776 (1995)) can result in leukocyte accumulation at the site of chemokine injection or expression. Antagonists of chemokines can prevent leukocyte trafficking (Bargatze, R. F. and Butcher, E. C., J. Exp. Med., 178:367–372 (1993)) and may have beneficial effects in several models of acute and chronic inflammation (Sekido, N., et al., Nature, 365:654–657 (1993); Karpus, W. J., et al., J. Immunol., 155:5003–5010 (1995)). Chemokines have also been reported to modulate angiogenesis (Gupta, S. K., et al., Proc. Natl. Acad. Sci. USA, 92:7799–7803 (1995) and hematopoiesis (Taub, D. D. and Openheim, J. J., Ther. Immunol., 1:229–246 (1994)), as well as T lymphocyte activation (Zhou, Z., et al., J. Immunol. 151:4333–4341 (1993); Taub, D. D., et al., J. Immunol., 156:2095–2103 (1996)). In addition, several chemokine receptors act as co-receptors, along with CD4, for entry of M tropic and T tropic HIV-1 (Choe, H., et al., Cell, 85:1135–1148 (1996);Feng, Y., et al., Science, 272:872–877 (1996)).
Leukocyte adhesion to endothelium is thought to involve several overlapping steps including rolling, activation and arrest. Rolling leukocytes are exposed to factors expressed at the adhesion site resulting in activation of the leukocyte and up-regulation of integrin-mediated adhesion. As a consequence of such integrin-mediated interactions, leukocytes arrest on the endothelium (Bargatze, R. F. and Butcher, E. C., J. Exp. Med., 178:367–372 (1993); Bargatze, R. F., et al., Immunity, 3:99–108 (1995)). Leukocyte activation and up-regulation of integrin molecules occurs via a pertussis toxin-sensitive mechanism that is thought to involve chemokine receptors (Bargatze, R. F. and Butcher, E. C., J. Exp. Med., 178:367–372 (1993); Campbell, J. J., et al., Science, 279:381–383 (1998)).
Memory CD4+ lymphocytes can be grouped based upon the expression of certain chemokine receptors. For example, CXCR3, CCR2 and CCR5 (Qin, S., et al., Eur. J. Immunol., 26:640–647 (1996); Qin, S., et al., J. Clin. Invest., 101:746–754 (1998); Liao, F., et al., J. Immunol., 162:186–194 (1999)) are all expressed on subsets of memory CD4 lymphocytes, and certain chemokines act selectively on naive T cells (Adema, G. J., et al., Nature, 387:713–717 (1997)). Furthermore, several chemokines which are ligands for such receptors have been shown to be expressed in inflammatory sites (Gonzalo, J. A., et al., J. Clin. Invest., 98:2332–2345 (1996)) and in some cases in lymph nodes draining a challenged site (Tedla, N., et al., J. Immunol., 161:5663–5672 (1998)). In vitro-derived TH1/THH2 lymphocyte lines have also been shown to differentially express chemokine receptors. Specifically, TH1 lymphocytes have been shown to selectively express CXCR3 and CCR5, while TH2 lymphocytes selectively express CCR4, CCR8 and CCR3 (Bonecchi, R. G., et al., J. Exp. Med., 187:129–134 (1998); Sallusto, F. D., et al., J. Exp. Med., 187:875–883 (1998); Sallusto, F., Science, 277:2005–2007 (1997); Andrew, D. P., et al., J. Immunol 161:5027–5038 (1998); Zingoni, A., et al., J. Immunol., 161:547–555 (1998)). Interestingly, in some cases the chemokines for these respective chemokine receptors, such as MDC for CCR4 and IP-10 for CXCR3, are induced by cytokines associated with a TH1/TH2 environment (Andrew, D. P., et al., J. Immunol 161:5027–5038(1998); Luster, A. D., et al., Nature, 315:672–676 (1985)).